The number and kinds of zoonotic helminths found in human tissues (cysts, abscesses, swellings, infarcts, tumor-like growths) have increased sharply in recent years. When an object found in histopathological sections is believed to be a part of a worm, or is recognized as such, there is an urge to make an exact identification of it so that its presence in the tissues can be explained and its significance in relation to disease can be understood. On present knowledge, relatively few kinds of worms can be identified in tissue sections even at a broad generic level. Because most identifications are inexact, and often incorrect, neither the range or the frequency of occult helminthic infection in man is well documented. However, detailed descriptions of features seen in sections have been made for a sufficient number of worms to demonstrate the feasibility of making accurate, definitive identifications based on representative microsections. Also, it has been observed that certain worms and larvae adapted to living in tissues induce a specific, non-inflammatory tissue reaction, each of a type peculiar to itself. The objectives in the proposed study are (1) to describe and identify a number of worms that have been found, and will be in the future found, in the tissues of human brain, eye, lung and other organs, and (2) through a series of descriptive studies on selected representative types, to develop a system of differential diagnosis based on histological and cellular features of worms seen in sections and, in special cases, on the character of the surrounding, non-inflammatory host tissue reactions induced by them, and (3) to gain a more exact knowledge of the behavior patterns of tissue-inhabiting helminth parasites of animals to better anticipate and interpret their localization and migratory behavior in man.